Wednesday, December 20, 2006

Trees + Wet + Wind = Blow Over

One of the most lasting effects of last week's wind storm will be the loss of numerous trees throughout the region. While the two big cottonwoods in our backyard only lost small branches, a lot of large trees in the area were completely uprooted and knocked over. One example is this large tree in the park near our house.

Sure, it was windy, but why did some trees fare worse than others?

  • Trees sway back and forth in the wind, as stress is loaded and unloaded from its trunk and branches. The mechanical properties of the tree (set by size, species, etc.) control the sway period, and trees with high heights relative to their basal diameter will be more susceptible to wind damage. So some species will tend to be more wind resistant than others, and big trees will respond differently than little trees.
  • As the tree roots move in response to the swaying, trees with shallow root systems will be more likely to pull out of the ground than deep rooted trees. So again, there is a species difference in the likelihood of toppling.
  • But even in trees than normally have deep root systems, if there is a shallow water table (i.e., it is saturated near the surface) at least part of the time, root systems will be a lot shallower, and there will be an increased probability of blow overs. Additionally, when soils are saturated, positive pore pressure reduces the cohesive strength of the soil, which probably makes it easier for tree roots to pull out of the ground.

  • I'd put the blame on a shallow water table for the demise of the tree in our park:

As you can see from the pictures, even though the tree was probably 40+ feet high, its major root system extended less than 2 feet below the ground surface, and the water table (post-storm at least) is even shallower.


Lab Lemming said...

Even if the water table isn't shallow, cultivated trees planted in parks (at least since the 1950's) are disproportionately swamp trees, which have evolved shallow root systems from millions of years of growing close to the water table.

The reason that swamp trees are popular for planting is simple: Trees without tap roots can be transplanted at a larger size, and more cheaply, than trees with them.

Here in Canberra, all the pin oaks and elms have dead upper branches from the 2003 drought. the locals give this as a reason that imported trees shouldn't be used for landscaping. But chestnut oaks and ponderose pine probably would have fared just fine.

Anonymous said...

This is fascinating! Thanks for posting a nice explanation. I'd like to hear more about this..especially with respect to various species. Where did you get this info? Should I google it?

girl with trees & goats

ScienceWoman said...

Lab lemming - thanks for the info. I've been meaning to go get some leaves and identify the tree - but with the short days, I'm having trouble getting there in the daylight.

Girl with trees and goats - A little bit of stuff in my head + Google. Most of the top results though came from the SE US rather than the PNW. I guess near-annual hurricanes vs. freak windstorms make a difference in what is on people's minds.